Tapping Drill Sizing
The correct size of tapping drill can be calculated using the following formula.
A thread depth between 63% and 83% will give satisfactory results. The use of 75% in the formula is suggested.
55 degree thread forms e.g. BSW, BSF (inch calculation)
Drill Size = Nominal Tap Diameter - (1.28 / TPI x % of thread depth)
e.g. 5/16" BSF, 75% thread required
Drill Size = .3125" - (1.28 / 22 x 75 / 100) = .3125 - 0.0436
Drill Size = 0.2689"
For metric drill size multiply answer by 25.4
therefore Metric drill size = 6.83mm
nearest Standard Drill = 6.8mm
60 degree thread forms e.g. BSCY, UNF, UNC (inch calculation)
Drill Size = Nominal Tap Diameter - (1.30 / TPI x % of thread depth)
Metric (ISO)
Drill Size = Nominal Tap diameter in mm - pitch
e.g. M8 (1.00mm pitch)
Drill size = 8 - 1 = 7.0mm